Hydrogen gas is colorless and odorless. Hydrogen burns in air with an invisible flame in an outdoor setting under normal daylight conditions. Due to the small size of the hydrogen molecule, it is extremely difficult to render mechanical joints leak free without welding. Some joints, such as those found at facilities where hydrogen is loaded and stored, must have removable joints for connection, e.g., to delivery vehicles. Similarly, at end-use sites, such as a launch vehicle, removable connections must exist to enable filling of onboard tanks. Hydrogen gas is commonly detected using fixed gas detectors. Outdoors, hydrogen is rapidly dispersed by moving air due to its low molecular weight and density. Further, hydrogen has a low ignition energy and a low threshold concentration, making hydrogen fires a significant hazard in such areas. The problem is further compounded for operations, such as launch complexes, where large quantities of hydrogen and oxidizer in close proximity dictates safety regulations that preclude the use of typical handheld leak or flame detectors by operators to confirm leaks or flames sensed by fixed instruments. Use of fixed leak detection instruments can be problematic due to the ease with which hydrogen disperses outdoors due to air currents. Depending on the location of the leak and the detector(s), the leak may need to be large in order to register on the detector.
For many applications, non-imaging flame detectors do not provide desirable features, such as flame size identification and localization, within a monitored area. Various approaches seek to detect flames and/or leaks using single spectrum, multi-spectrum, non-imaging and imaging devices. Such devices can utilize ultraviolet (UV), near infrared (NIR), or infrared (IR) detection approaches to image electromagnetic emission characteristics of flames in general or flames resulting from the burning of specific materials, such as carbon compounds in air. To date, while many current devices can effectively identify a flame in a monitored area, these devices are susceptible to false indications of flames, e.g., due to reflections of flames, sunlight (direct or reflected), and reflections from vegetation. As a result, operations personnel can be required to review imagery to formulate a correct response to the device identifying a fire in a monitored area. Additionally, such devices are not suitable for use in mission critical applications, such as rocket launch operations, due to the cost associated with mission aborts resulting from false alarms.
In an illustrative prior art flame detection approach, a multispectral method of flame detection employs three infrared detectors and associated filters to select portions of the infrared spectrum. A user uses the device of this approach in the manner of a binocular to view imagery based on spectral content in the near infrared (NIR) region of the spectrum below 800 nanometers (nm) or 1100 nm. The filters render invisible flames visible due to water emissions in the 850-1250 nm portion of the electromagnetic spectrum. The device can trigger an alarm for the user when a flame is detected.
In another illustrative prior art flame detection approach, an imaging flame detection system employs a camera with an 1140 nm band pass filter to select emissions from flames. The system performs size and flicker analysis on blobs extracted from the imagery using a stored reference of flame and false alarm signatures to discriminate between a flame or a false alarm.
In still another illustrative prior art flame detection approach, a non-imaging approach to flame detection utilizes two infrared sensors and one ultraviolet sensor. Cross correlation between the infrared and ultraviolet signals is performed to discriminate between a flame and a reflection of a flame.
The sound made by a pressurized gas or liquid escaping from an orifice is determined by the source pressure and the size of the orifice, which serve to generate turbulence in the air in the immediate vicinity of the leak or flame. Additionally, the resulting turbulence is also dependent on the particular material of the leak or flame. The turbulence, which results in rapid pressure fluctuations in the air near the leak or flame source can be detected with appropriate acoustic pressure transducers. Pressure fluctuations due to a leak or flame typically have a broad spectral content with maximum intensity in the ultrasonic portion of the spectrum (e.g., 20-50 kHz). Ultrasonic energy experiences significant atmospheric attenuation as it emanates from a source, e.g., typically approximately twenty-five decibels per one hundred feet at twenty kilohertz. The attenuation increases with the second power of distance due to circular spreading and with the second power of frequency due to atmospheric absorption.
Devices to detect ultrasonic waves have been proposed for leak detection at distances up to twenty-five feet in a normal outdoor acoustic environment. In an illustrative prior art leak detection approach, an omni-directional microphone detects ultrasonic signals, which are processed using amplitude and temporal duration thresholds to identify leaks from background noise.
Reflecting concave surfaces, such as spherical or paraboloid sections, can focus energy incident on the surface to a single point, referred to as the focal point. Such surfaces are often used as radar and satellite dish antennas to focus electromagnetic waves and for surveillance and tracking certain animals using acoustic waves. Some approaches to leak detection have proposed the use of parabolic antennas to assist in localization of the leak source.